1. Field of the Invention
The present invention relates to an image projection apparatus for projecting a high-definition picture image in a wide angle of view onto a retina.
2. Description of the Related Art
Hitherto, this kind of apparatus has been designed so as to become small and thin so that it can be used while being mounted on a human head. As an example, Japanese Patent Unexamined Publication No. Hei. 2-136818 discloses an image display apparatus. This apparatus is such that a picture image displayed on a display element is made to bypass an optical system of an eye of an observer, so that it is directly projected on a retina. Thus, in this apparatus, it is possible to observe an image in an angle of view corresponding to a converging angle of a lens. According to this method, it is not necessary to add a function such as diopter adjustment to the optical system. This apparatus is specifically constituted by a point light source such as a midget lamp, a picture image panel irradiated by this point light source, and a short focus convex lens integrated with this picture image panel. In this apparatus, an image on the picture image panel illuminated with light from the point light source is brought into focus at a pupil positioned in an eyeball lens. This focus is a light source image and becomes a diffraction image plane of the picture image displayed on the display element. This point is made a pinhole, and the picture image on the display element is directly projected on the retina. In this apparatus, although it is possible to observe an image in an angle of view corresponding to a converging angle, large aberration occurs in the case where only one short focus lens is used, so that it is impossible to observe a high-definition picture image.
An image display apparatus improving this point is disclosed in, for example, Japanese Patent Unexamined Publication No. Hei. 4-23579 (or U.S. Pat. No. 5,371,556). This apparatus is constituted by an illuminating lens for illuminating a picture image panel, and an objective lens for projecting a picture image of a display element onto a retina. Specifically, after an angle of divergence of a beam of light from a midget lamp (point light source) is once controlled by a lens, the direction of the beam of light is changed by two mirrors, and then, the beam is made a parallel beam by a convex lens to illuminate an LCD panel. The direction of the beam of light is controlled by a convex lens disposed immediately after the LCD panel and the picture image beam of light is condensed by a short focus convex lens. An image of the point light source is formed on the pupil of an observer, so that the picture image of the display element is projected on the retina of the observer.
Japanese Patent Unexamined Publication No. Hei. 5-328261 (or U.S. Pat. No. 5,499,138) discloses another improved image display apparatus. In this apparatus, a two-dimensional microlens array is disposed on a display element, and an image of a pixel formed there is formed on a retina. Specifically, the two-dimensional microlens array is disposed on a transmission liquid crystal display element illuminated by a two-dimensional LED array or surface light source, and an outgoing beam of light from each pixel of the display element is converted into a parallel beam of light by a condenser lens and is led into an eyeball. In this apparatus, since an image corresponding to each pixel is formed on the retina by the lens action of the eyeball, a clear image can be obtained.
On the other hand, U.S. Pat. No. 4,309,070 discloses a display apparatus using a large-off axis hologram optical element to miniaturize a display apparatus of a beam combiner. In this apparatus, a beam of light of a picture image displayed on a screen of a CRT is converted into a collimated beam of light through a lens, is incident from an end surface of a transparent substrate, is reflected in the substrate, and is diffracted by the hologram. Although an observer observes a virtual image, the observer can observe the outside at the same time through the transparent substrate and the hologram.
Among the related art, in the apparatus using the illuminating lens and the objective lens, as compared with an apparatus including only one short focus lens, an image with high resolution can be obtained. However, there has been a problem that the resolution is influenced by the size of the light source. Besides, in the apparatus using the illuminating lens and the objective lens, although it is possible to observe an image in a wide angle of view, at least two lenses are required and the optical system becomes complicated. Actually, for the purpose of correcting (color) aberration and spherical aberration, a combination lens of at least four lenses is required. Basically, the lens is used in a coaxial optical system. In order to prevent the size of the apparatus from increasing, contrivance of use with folding at a mirror is adopted. However, the number of optical parts becomes eight, and it has been difficult to miniaturize the apparatus any more.
In the apparatus using the two-dimensional microlens array, as compared with an apparatus of using only one short focus lens, an image corresponding to each pixel is obtained, so that clear display can be obtained. However, an optimum pixel size and a pixel pitch obtained from calculation are 6.5 xcexcm square and 185xc3x97184 xcexcm. That is, although the pixel size is small, the pixel pitch is large. For the purpose of obtaining high resolution, a display element of a large screen becomes necessary, and the increase of the size of the apparatus can not be avoided. Even if a large angle of view is obtained, since parallel beams of light are led into the eye and the optical system of the eye is used, the merit that the optical system of the eye can be bypassed is lost differently from the structure of the apparatus using only one short focus lens.
In the apparatus using one short focus lens or the illuminating lens and the objective lens, when the movement (direction of line of sight) of the eye of the observer and the mounting position of the apparatus are changed, distortion occurs in the image formed on the retina. The reason is that although the picture image panel and the retina are in conjugate relation through the large pinhole of the pupil, when the position of the line of sight and the apparatus is shifted, the lens function of the eyeball contributes. In order to solve this problem, such contrivance has been made that a focal position is controlled by a mechanical deviation so that the focal position at the rear side of the short focus convex lens comes to the center of the eyeball lens. However, the increase in the size of the apparatus itself can not be avoided also in this case.
On the other hand, the apparatus using the large-off axis hologram has been proposed for miniaturizing a beam combiner display apparatus. However, when a virtual image of the CRT image is observed, the position of the virtual image and the size thereof are limited. Particularly, since the light path difference between the display and the large-off axis hologram optical element is greatly changed according to the position by the large-off axis, aberration becomes apt to occur. Like this, although this apparatus can be made compact, there are problems that an angle of view is narrow and the resolution becomes low.
From the above, the problem of an image projection apparatus is to realize such an apparatus that
1) the apparatus is miniaturized (including a decrease in the number of parts) while maintaining high resolution and a wide angle of view,
2) the outside can be perceived at the same time, and
3) distortion does not occur on the retina image by a small shift at the time of mounting, and so on.
An object of the present invention is therefore to provide an image projection apparatus which is small and in which a picture image can be stably observed while high resolution is maintained. Another object of the present invention is to provide an image projection apparatus having a see-through property in addition to the foregoing object.
The foregoing objects are achieved by an image projection apparatus which comprises a light source, a space filter receiving a beam of light from the light source to function as a secondary light source, and a space modulator illuminated with the beam of light from the secondary light source, and in which an image of the secondary light source is formed on a pupil and a picture image of the space modulator is projected on a retina.
Here, an illuminating optical system may be provided between the space filter and the space modulator.
The illuminating optical system is constituted by a hologram optical element or a diffraction optical element. Besides, an object optical system may be provided at an outgoing beam side of the space modulator. The object optical system is also constituted by a hologram optical element. The hologram optical element is of a large-off axis type, and is made of a transparent material to secure a see-through property.
The hologram optical element can be manufactured so as to correct color aberration and spherical aberration. Thus, one hologram optical element can be substituted for two lenses, so that the number of optical parts can be reduced. The hologram optical element has such uses as a diffraction grating, a lens, a beam combiner, a beam splitter, a filter, and a scanner according to a forming method of interference fringes. When the hologram optical element is used as the beam combiner, it is possible to see both the bright outside and the image display in overlapping. Since the function of the beam combiner can be added to the hologram optical element, the number of optical parts can be reduced.
In order to transmit an illuminating beam of light, a space modulation beam of light, and the like with high resolution by using the hologram optical element, as a result of optical simulation, it has been found that a coherent illuminating system is suitable. In order to obtain an illuminating beam of light having high coherence, it is appropriate that a spherical wave from a secondary light source is used. Here, the secondary light source is a light source image formed with a beam of light from some light source through, for example, a condensing optical system. Since this secondary light source is minute, it will be also hereinafter referred to as a secondary point light source or minute secondary point light source. This kind of optical system is similar to an optical system used in an object optical system of a microscope.
When the minute secondary point light source is used, an image formed on the pupil becomes a secondary point light source image. As a result, since the large pinhole of the pupil is not used but the light source image itself is minute, the depth of focus becomes deep. Thus, even if a small shift occurs at the time of mounting, the retina image is not distorted, but can be stably produced. The secondary light source is formed by the space filter. The space filter has an opening with a diameter of 2 mm or less, preferably 25 xcexcm or less. Since a human pupil is about 2 xcexcmm at a bright place, the opening of the space filter is made smaller than this.
An image projection apparatus of the present invention can be structured into a head mount type or an eyeglasses type, and includes an optical source disposed at a side of a frame of eyeglasses, a space filter receiving light from the light source to function as a secondary light source, a space modulator illuminated with light from the secondary light source, a transparent substrate attached to the frame of the eyeglasses, and a hologram optical element disposed on the transparent substrate to be adjusted such that an image of the secondary light source is formed on a pupil and a picture image of the space modulator is projected on a retina. Here, the transparent substrate may be made to have a flat surface shape or curved surface shape. The hologram optical element is formed by using any one of silver salt film, dichromated gelatin, and photopolymer. It is preferable that this hologram optical element is covered with a protective film.
Moreover, an image projection apparatus of the present invention includes a light source, and a space modulator illuminated with a beam of light from the light source, and is structured such that the diameter of a zeroth-order diffraction beam of light and a first-order diffraction beam of a diffraction image surface formed on a pupil by the beam of light from the space modulator is made 2 mm or less, and a picture image of the space modulator is projected on a retina. Here, as the light source, a secondary light source formed by the space filter can be used. The space filter is formed of at least one of an aperture stop and a field stop or a pinhole.
Moreover, an image projection apparatus of the present invention includes a light source in which the diameter of an outgoing beam of light is 2 mm less, and a space modulator illuminated with the beam of light from the light source, and is structured such that an image of the light source is formed on a pupil and a picture image of the space modulator is projected on a retina. Alternatively, an image projection apparatus comprises a light source, a space filter having an opening with a diameter smaller than the diameter of an incident beam of light from the light source, and a space modulator illuminated with the beam of light having passed through the space filter, and is structured such that a condensed point of light from the space modulator is positioned on a pupil, so that a picture image of the space modulator is projected on a retina.
According to an image projecting method of the present invention, a beam of light from a light source is made to pass through a minute opening, a space modulator is illuminated by using the minute opening as a secondary light source, an image of the secondary light source is made to be formed on a pupil, and a picture image of the space modulator is formed on a retina. A hologram optical element is used to form the image of the secondary light source.
By the structure as described above, according to the present invention, it is possible to obtain an image projection apparatus which is small and in which a picture image is stably observed while high resolution is maintained. At the same time, a see-through property can be secured.